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Journal of Autism and Developmental Disolders, Vol. 23, No. 1, 1993

Self-Injurious Behavior in Rett Syndrome: Interactions Between Features of Rett Syndrome and Operant Conditioning 1

Chris Oliver, 2 Glyn Murphy, and Lissa Crayton Institute of Psychiatry, University of London

John Corbett Department of Psychiatly, University of Bbmh~gham

Operant and biological theories of the cause of self-injurious behavior (SIB) in people with a mental handicap are often viewed as mutually exclusive. In this single case study, interactions between features of Rett syndrome and operant conditioning as determinants' of SIB are examined. Functional analysis by analog methodology indicated different functions for two forms of SIB shown by the subject: automatic' rehTforcement by sensory stimulation and escape from social interactions. It is suggested that features of Rett syndrome established conditions under which operant conditioning of self-injurious responding was maximized. The implications of this interaction between features of syndromes" and operant conditioning for the conceptualization of the cause of SIB are discussed and it is proposed that the notion of a unitary cause of SIB is inappropriate. It is mole productive to consider operant conditioning as the process that maintains responding against a background of predisposing and mediating factors which may be biologically determined.

IThis research was funded by the Department of Health. We would like to thank the staff at Hilda Lewis House for their participation in this study and June Heffernon for typing this manuscript.

2Address all correspondence to Chris Oliver, Department of Psychology, Institute of Psychiatry, DeCrespigny Park, London, SE5 8AF, England.

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0162-3257/93/0300-0091507.00/0 �9 1993 Plenum Publishing Corporation

92 Oliver et al.

INTRODUCTION

Self-injurious behavior (SIB) in people with a mental handicap occurs in approximately 12% of people living in institutions and between 4 and 8% of the total population (Jacobson, 1982; Kebbon & Windahl, 1986; Oliver, Murphy, & Corbett, 1987). The presence of self-injury is generally positively associated with a more severe degree of mental handicap (Kebbon & Windahl, 1986; Rojahn, 1986), sensory and physical impair- ments (Kiernan & Kiernan, 1987; Schroeder, Schroeder, Smith, & Dalldoff, 1978), and expressive communicative deficits (Ando & Yoshimura, 1979a, 1979b; Carr & Durand, 1985a; Shodell & Reiter, 1968). There is also an association between SIB and particular syndromes such as Lesch-Nyhan (Christie, et al., 1982), Gille de la Tourette (Robertson, Trimble, & Lees, 1989), and Prader-Willi syndromes (Clarke, Waters, & Corbett, 1989), and syndromes associated with congenital or acquired insensitivity to pain (Altman, Haavik, & Higgins, 1983; Itoh, Yagashita, & Nakajima, 1986). De Lange syndrome has also often been purported to have an association with SIB (Johnson, Ekman, Friesen, Nyhan, & Shear, 1976; Shear, Nyhan, Kirman, & Stern, 1971) but this is currently the subject of debate (Oliver, 1988; Russo, Carr, & Lovaas, 1980).

Recently the presence of SIB has been reported in Rett syndrome (Hagberg, Aicardi, Dicas, & Ramos, 1983; Iwata, Pace, Willis, Gamache, & Hyman, 1986; Olson & Rett, 1985). This syndrome, which only occurs in females, is a degenerative disorder characterized by severe or profound mental handicap, apraxia of gait, stereotyped use of the hands, loss of facial expression, and a period of autistic-like withdrawal. The degeneration be- gins towards the end of the first year or early in the second year of life and has a progressive course, but only for a year or so (Coleman, Brubaker, Hunter, & Smith, 1988; Hagberg et al., 1983). The cause is unknown and the incidence is estimated to be 1 in 30,000 births (Kerr & Stephenson, 1985). Although SIB is not consistently reported, a number of studies note the presence of rapid hand to mouth movements which may cause abrasions or lesions of the skin (Hagberg et al., 1983; Olson & Rett, 1985).

The link between syndromes and SIB has stimulated interest in the potential neurochemical determinants of SIB; more specifically disturbance of the opiatergic, serotonergic, and dopaminergic systems (Winchel & Stanley, 1991). It has been proposed, for example, that SIB in Lesch-Nyhan syndrome is related to dopamine receptor supersensitivity (Baumeister, Frye, & Schroeder, 1984) and dopamine disturbance is also implicated as a contributory factor to SIB in Gille de la Tourette syndrome (Gillman & Sandyk, 1985). These theories are predominantly based on animal models (e.g., Mueller & Nyhan, 1983), the use of dopaminergic antagonists

Self-Injurious Behavior in Rett Syndrome 93

(Goldstein, Anderson, Reuben, & Dancis, 1985), and neuropathological findings (Baumeister et al., 1984). One implication of these theories is that they may have a more general application to people who show severe SIB but do not have any of the syndromes discussed above and there is some evidence that this may be the case (Gualtieri, Schroeder, Keppel, & Breese, 1986; Oliver & Head, 1990).

Theories of the neurochemical determinants of SIB stand in stark contrast to the operant theory which asserts that SIB is behavior governed by environmental events (Carr, 1977; Johnson & Baumeister, 1978). The operant theory has gained empirical support via studies of stimulus control (Carr, Newsom, & Binkoff, 1976; Edelson, Taubman, & Lovaas, 1983; Murphy & Oliver, 1987), the manipulation of reinforcers (Carr & McDow- ell, 1980; Lovaas, Freitag, Gold, & Kassorla, 1965), and the covariation of SIB with other behaviors that have functional similarity (Carr & Durand, 1985b). This research has shown that SIB may be maintained by contingent positive reinforcement such as attention or more tangible stimuli presented by others (Durand & Crimmins, 1988), negative reinforcement by contingent escape from demands or instructional settings (Gaylord-Ross, 1982; Iwata, Pace, Kalsher, Cowdery, & Cataldo, 1990), and automatic reinforcement (i.e., either positive or negative reinforcement; see Iwata et al., 1990) by contingent sensory stimulation (Meyerson, Kerr, & Michael, 1967; Rincover & Devany, 1982).

These areas of the study of SIB (operant and neurochemical theories) are commonly viewed as mutually exclusive, or at least competing, in terms of "the cause" of SIB. Such an approach is both unproductive and inappropriate as interactions may exist between these seemingly unrelated determinants of SIB (Oliver & Head, 1990). Anderson, Dancis, & Alpert (1978) for example, described SIB in Lesch-Nyhan syndrome to be both determined by environmental contingencies and responsive to a behavioral intervention. In this case, while SIB may have initially been related to neurochemical concomitants of Lesch-Nyhan syndrome, clearly an operant explanation was more apposite at a later point (see Murphy & Wilson, 1985). There is therefore good reason to suspect that interactions between features of syndromes and operant contingencies occur and determine aspects of SIB. While at present the main feature of syndromes which has been considered is neurochemical disturbance, there may be behavioral features of syndromes which are equally important as contributory determinants of SIB but have yet to be described. These features may require attention because of the implications for interventions.

To examine interactions between features of syndromes and operant conditioning it is necessary to evaluate both the extent of effects of the im- mediate environment on SIB and the operative mode of reinforcement. This

94 Oliver et aL

may be done by conducting a functional analysis of the behavior and describ- ing the influential antecedents and consequences (Blackman, 1985; Kiernan, 1973). One method of functional analysis developed recently is that of analog methodology (Cart & Durand, 1985b; Iwata, Dorsey, Slifer, Bauman, & Rich- man, 1982). This method involves exposing the individual to controlled environmental conditions in which putative, influential antecedent, and consequent events are systematically varied. The effect of these conditions on the rate of responding is observed and inferences may then be drawn on the antecedent and reinforcing contingencies that exist in the natural environment (see Oliver, 1991). Using this method it is possible therefore to evaluate both the extent of environmental influence (by examining the overall effect of changing conditions on responding) and the operative mode of reinforcement (by comparing the effects of different conditions).

In this paper the potential interactions between features of Rett syndrome and operant conditioning are considered. Evidence for operant maintenance of self-injurious responding is presented based on a functional analysis employing analog methodology and related to particular features of Rett syndrome which have been outlined above.

METHOD

Subject

N was a nonambulatory 3.5-year-old girl with Rett syndrome who attained a mental age equivalent of approximately 6 months on the Bayley Scale of Infant Development. Her early developmental milestones were described as normal until the age of 16 months when delay in speech and motor abilities became apparent. Purposeful hand use also deteriorated and she exhibited stereotyped hand-wringing movements with the hands most commonly held together in the midline position, She was unable to stand unsupported. Self-injurious behavior began at approximately 2 years. When she was admitted as an inpatient to Hilda Lewis House (a short-stay unit) she was causing herself tissue damage by repeated SIB which took the form of rapid hand to mouth movements. Rett syndrome was diagnosed (by J.A.C.) and functional analysis of her behavior began shortly afterwards.

Procedure

N was exposed to four different experimental analog conditions during sessions. There were eight sessions comprising the four conditions


spread over 6 days and one condition only in the ninth session on the 7th day. The intention of employing this methodology was to elucidate conditions under which SIB occurred and therefore it had to be expected that some conditions wouM result in SIB. However, all conditions were essentially replicating situations that might occur on an everyday basis and none lasted more than 15 minutes. Where the SIB appeared likely to cause tissue damage, sessions were immediately stopped. The number of sessions was kept to a minimum, the main goal being to use the information gained to design an effective intervention.

The order of the conditions within a session was random and determined before the experiment began. Each condition lasted for approximately 15 minutes, although some were stopped after a shorter time (see Results). A camera operator was in the room for all conditions but did not interact with N. Between conditions there was break of approximately 1 minute while the condition was changed. The subject was always seated on a soft mat on the floor, she was never restricted from emitting a self- injurious response, and there were no contingencies for any self-injurious response. Four adults (three nurses and a psychologist) participated as the single adult in the relevant conditions, each being present in at least 2 sessions. The four conditions were as follows:

Continuous Attention. The adult sat with the child and was instructed to give the child his/her full attention in a "pleasant undemanding way." This involved singing to the child, talking, playing finger games, and so forth. No toys or objects were available for the child to play with or touch throughout this condition.

No-Stimulation Condition. The child was in the same situation except the adult was not in the room. The only person in the room was the camera operator. There were no toys or any objects within the child's reach and the room was kept as quiet as possible.

Stimulation Condition. The child was given toys which she was known to enjoy mouthing and manipulating. These included soft plastic rings, squeaky rubber toys, pegboards of various sizes, and cuddly toys. No adult sat nearby in this condition (only the camera opera tor was in the room).

Demands" Condition. An adult sat next to the child and required her to carry out three separate tasks for 5 minutes each. The order of the tasks was decided randomly and all three tasks (a small pegboard, a large pegboard, and stacking rings) had been present in the stimulation condition. They were all known to be too difficult for the child to complete independently. The participating adult was instructed to prompt the child ver- bally, gesturally, and physically to carry out the task. Adults were told when to change the task (every 5 minutes) and this was done without a break.

96 Oliver et al.

The task and demands were not removed until the end of the condition, that is, they were not removed contingent on self-injurious responding (cf. Iwata et al., 1982).

The rationale for employing these conditions was that: 1. If SIB was maintained by the positive reinforcement of contingent

attention from others then SIB would be higher in conditions in which an adult was not present (i.e., during both "stimulation" and "no stimulation") as an establishing operation of reinforcer deprivation would exist (see Carr & Durand, 1985b; Hall & Oliver, 1992; Michael, 1982). (An establishing operation has been defined by Michael, 1982, as "any change in the environment which alters the effect iveness of some object or event as r e i n f o r c e m e n t and s imul taneous ly al ters the momen ta ry f requency of the behavior that has been followed by that reinforcement.")

2. If SIB was maintained by the resultant sensory stimulation then SIB would be higher in the condition in which no stimulation was available (i.e., during "no stimulation") as there would be stimulatory reinforcer deprivation.

3. If SIB was maintained by escape from instructional demands then SIB would be higher in the condition in which demands were presented (i.e., during "demands") than the condition in which no demands were presented but someone was present (i.e., during "continuous attention").

4. If SIB was maintained by escape from social interactions regardless of whether demands were presented, the SIB would be higher in conditions in which others were present (i.e., during both Demands and Continuous Attention).

Recording Technique, Response Definitions, and Interobserver Reliability

All sessions were videotaped for later analysis. Data were collected on chosen behaviors using an Epson HX-20 lap-held microcomputer. The software (Repp, Harman, Felce, Van Acker, & Karsh, 1989) allows up to 43 separate behaviors to be recorded simultaneously in continuous time and thus avoids any form of sampling. Behaviors can be defined for either event or duration recording, as appropriate, and the raw data saved for later analysis. All data were transferred from the Epson HX-20 to an Acorn BBC Master Microcomputer for analysis using software written by the first author. Four subject behaviors were defined as follows:

Self-lnjurious Behavior in Rett Syndrome 97

Mouth hits. Any vigorous contact between the back of the hand and the mouth or cheek, when the hands were not previously together in the midline position. (Recorded as events)

Mouth flicks. Any rapid hand to mouth movement starting from the hands being held together in the midline position and resulting in contact between the back of the hand and the mouth. (Recorded as events)

Object touch. Any active manipulation of objects by the subject, excluding resting hands or arms on an object. (Recorded as durations)

Object mouth. Any contact between objects and the subject's mouth. (Recorded as durations)

Two adult behaviors, which were only observed in the demand conditions, independently of any subject behaviors, were defined as follows:

Adult vocalization. Any adult vocalization. (Recorded as durations) Adult contact. Any physical contact between the adult and child which

was initiated by the adult. (Recorded as durations) Interobserver reliability was evaluated for the four subject behaviors

and two adult behaviors for 18.75 and 25% of all relevant sessions, respectively. Agreement was calculated for each subject behavior on a 10-second interval-by-interval basis and a 5-second basis for adult behaviors. Mean Cohen's kappa indices of agreement (see Murphy, 1987) were 0.69 for mouth hits (range = 0.50-0.83), 0.78 for mouth flicks (range = 0.50-0.89), 0.78 for object touch (range = 0.75-0.80), 0.82 for ob jec t m o u t h (range = 0.72-0.88), 0.71 for adult vocalization (range = 0.66-0.76), and 0.74 for adult contact (range = 0.69-0.79). Levels of interobserver reliability for all behaviors were adequate and all mean kappa values were above 0.60. Thus it appears that observers could satisfactorily distinguish between the two somewhat arbitrarily differentiated behaviors of mouth flicks and mouth hits, even though the definitions had only subtle differences.

RESULTS

Each condition was to have lasted 15 minutes but on three occasions (Sessions 2, 6, and 8) it was deemed necessary to terminate a condition after 5 minutes due to the subject's distress and the possibility of tissue damage occurring. On all three occasions it was the continuous attention condition that elicited the very high rates of SIB and the distress. Further- more in the ninth session, which started with the continuous attention condition, it was decided to terminate the experiment at the end of the condition because of the high rates of SIB and the distress shown by the subject.

98 Oliver et al.

Figure 1 summarizes the results and presents the overall mean frequency per minute of flicks and hits for each condition together with 1 standard error.

From Figure 1 it is apparent that face hitting occurred predominantly in the continuous attention condition. Flicks however, appeared at the highest rate in the no-stimulation condition, although the difference between the rate in the no-stimulation condition and the other conditions is not pronounced. In the stimulation condition object touch occurred for 23.9% of the time and object mouthing for 10.4% of the time. In the no-stimulation condition these behaviors occurred for 2,1 and 1.6% of the time, respectively.

To explore the trend in N's behaviors within conditions the mean frequency per minute of flicks and hits was calculated for each minute, within each condition, across all trials of a condition. As these data were to be inspected for a trend across time, a 3-point moving mean was applied to the means for each minute and the results are shown in Figure 2.

The data presented in Figure 2 show that in both demands and continuous attention conditions the rate of hits rises within the condition. (This effect may have been more pronounced if some trials of the continuous attention condition were not terminated before 15 minutes had elapsed.) This pattern is not evident for the flicks in these conditions or for hits in the other two conditions. It is also apparent that at the point at which the hits begin to rapidly increase (Minutes 8 to 10 in the continuous attention condition and Minutes 9 to 11 in the demands condition) the flicks begin to decrease. With regard to the flicks, the rate of responding is comparatively stable throughout all conditions except the stimulation condition where an increase in rate is evident toward the end of the condition.

Finally, in order to examine the variability between different trials of the demand condition, mouth hitting was plotted as a function of the percentage of time in the demands condition that the adult spent vocal- izing and the mean duration of adult physical contact in the demand condition. Scattergrams together with lines of best fit (determined by the method of least squares) and the results of Pearson correlations are shown in Figure 3.

Mouth hitting is significantly, positively associated with both the mean duration of adult physical contact, r(6) = .63, p < .05, and adult vocalizations, r(6) = .65, p < .05. The variability in mouth hitting therefore is to some extent determined by adult behavior within this condition.

Self-lnjurious Behavior in Rett Syndrome

FLICKS

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Fig. 1. Overall mean frequency per minute of flicks and hits shown by N in each analog condition. The standard error is also shown.

100 Oliver et al.

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Fig. 2. Overall mean frequency per minute of flicks and hits shown by N, over time, within each analog condition expressed as a 3-point moving mean.

Self- lnjurious Behavior in Rett Syndrome

DEMANDS CONDITION

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102 Oliver et al.

10

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Fig. 3. Mean frequency per minute of mouth hits shown by N in 8 sessions of the demand condition as a function of the mean duration of adult physical contact and the percentage of time that the adult vocalized.


DISCUSSION

From Figure 1 it is clear both that the analog conditions affected the occurrence of SIB and that the conditions affected mouth flicks and mouth hits differently (cf. Durand, 1982). (See Method for definitions of flicks and hits.) For the discussion of the functional significance of SIB the flicks and hits are therefore considered separately.

Mouth flicks occurred at a higher rate in the no-stimulation condition than any other condition in which other forms of stimulation were available. The stimulation condition appeared to provide stimulation as toy manipulation occurred for 23.9% of the time and toy mouthing for 10.4% of the time as opposed to the much lower levels in the no-stimulation condition thus suggesting there was a valid distinction between the conditions. The higher rate of flicks in the no-stimulation condition suggests that the flicks were automatically reinforced by their sensory consequences. An increasing rate of the flicks throughout the stimulation condition was evident (see Figure 2). This may be interpreted either as a scalloped curve indicative of fixed interval escape responding (see below) or as indicating the subject satiated to the forms of stimulation available and consequently responded to gain an alternative form of sensory stimulation. Given the overall higher rate of flicks in the no-stimulation condition as opposed to the stimulation condition this latter hypothesis seems more tenable.

This conclusion, that these rapid hand to mouth movements serve a sensory stimulation function in Rett syndrome, has also been drawn by Iwata et al. (1986) and Swahn (1988). Further evidence for this hypothesis comes from the anecdotal observation that much of N's "play" behavior consisted of vigorously mouthing play materials. The stimulation from these acts is presumably very similar to that gained from the hand to mouth movements. An alternative hypothesis is that the flicks are "organically" determined, as Rett syndrome is characterized by hand to mouth movements (Hagberg et al., 1983), and they may occur at a high rate when no incompatible behaviors (e.g., toy manipulation) are possible.

On the basis of these data the intervention for flicks consisted of en- suring that an appropriate form of stimulation was constantly available to N. This consisted of providing a "mouthing ring" on a necklace. This was chosen as it was appropriate to N's age and anecdotal observation of her play behavior (i.e., vigorous mouthing of objects) suggested that this form of stimulation was reinforcing (see Murphy & Oliver, 1987).

Although topographically similar the mouth hits seem functionally dissimilar by virtue of the different effect of the conditions. They occur at the highest rate in the continuous attention condition and this suggests they are maintained by escape from social interactions (see Procedure). Additional

104 Oliver et al.

evidence for this interpretation comes from the increasing rate of mouth hitting within continuous attention conditions shown in Figure 2 which ap- proximates to a scalloped curve indicative of fixed interval escape responding (Azrin, Hake, Holz, & Hutchinson, 1965; Hineline & Rachlin, 1969). Carr et al. (1976) have argued that this pattern is indicative of escape responding in demand settings and it is equally plausible that such a pattern should appear in other aversive settings where the experimental paradigm is similar. It is also interesting to note in the continuous attention condition that there is an increase in flicks and a subsequent decrease that coincides with a rapid increase in the hits. This may suggest that the increase in rate is also accompanied by an increase in intensity in the SIB ultimately with a change in topography. Other evidence, albeit subjective, which supports the hypothesis that the function of hits is to escape from social interactions, is that it was deemed necessary to terminate three trials of the continuous attention condition because of very high rates of hitting and extreme distress and eventually to terminate the experiment following the final continuous attention condition.

If the hits are maintained by escape from social interactions it is per- haps contradictory that they did not occur at an equally high rate in the demands condition, as demands essentially comprise a social interaction. From Figure 2 it is clear that the hits increase within demand conditions (and thus approximate to a scalloped curve indicative of fixed-interval escape responding) and this is consistent with the escape hypothesis. A possible explanation of the overall lower rate of hitting in the demands condition lies in the way in which the adults behaved in the condition and the nature of the condition itself. First, Figure 3 shows that two important variables in this condition are the percentage of time the adult spends vo- calizing and the mean duration of adult physical contact, and these may be some indicator of how "demanding" the adults were (cf. Romanczyk, Colletti, & Plotkin, 1980; Russo, Cataldo, & Cushing, 1981; Weeks & Gay- lord-Ross, 1981). Consequently, it may he that low rates of hitting occurred because some adults were not very demanding. Second, because of the nature of the condition it was possible for the adults to spend some time attending to the task and thus not to N. These two factors therefore may have to some extent "diluted" the social interaction.

On the basis of this formulation of the function of N's hits the intervention consisted of graded exposure to adult contact (by manipulating proximity and vocalizations) and positively reinforcing the absence of hits within this paradigm with edible reinforcers paired with minimal social contact. Contingent on hits, brief physical restraint and a verbal reprimand were employed (see Murphy & Oliver, 1987). While this latter contingency is essentially contingent attention and has been demonstrated to positively

Self-Injurious Behavior in Rett Syndrome lOS

reinforce SIB (Lovaas et al., 1965), the procedure would prevent further injury and the functional analysis indicated this contingency would not be reinforcing (see Figure 3).

From an operant perspective there is evidence that for N hits had the function of terminating social interactions while flicks were automatically reinforced by the resultant sensory stimulation. It is clear therefore that an operant interpretation is appropriate and that environmental factors are influential. This conclusion need not be viewed as inconsistent with studies reporting a higher prevalence of SIB in Rett syndrome and where the implication is that environmental factors are not influential. These positions are not irreconcilable if features of Rett syndrome are considered to raise the predisposition for the development of SIB and mediate operant processes that maintain responding.

Considering the flicks from this perspective, these are typical of the rapid, hand to mouth stereotypies seen in Rett syndrome. While this suggests limited operant influences, this inference is inconsistent with the data presented here and by Iwata et al. (1986) and the effective application of sensory extinction (an operant intervention) reported by Swahn (1988). These findings indicate automatic operant reinforcement by sensory stimulation. These positions may be reconciled by proposing that these hand to mouth stereotypies are not determined by a movement disorder, as a feature of Rett syndrome, but by an increased susceptibility (see Skinner, 1974) to the contingent sensory reinforcement most economically attained by the hand to mouth movement. Such an interpretation is consistent with both sets of findings and the anecdotal observation of the subject's play behavior (i.e., vigorous mouthing of objects).

Considering the hits, which functioned to terminate social interactions, the influence of Rett syndrome may be twofold. First, by virtue of the process described above rapid hand to mouth movements became domi- nant in the behavioral repertoire of N. Second, the degenerative process and period of autistic withdrawal characteristic of Rett syndrome led to social interactions being aversive to the subject (almost by definition, see Coleman et al., 1988). Other features of the syndrome, such as physical impairment and language loss, handicapped N in her ability to terminate social interactions appropriately. A behavior such as potentially damaging SIB which terminated interactions may therefore be functionally adaptive. An alternative hypothesis of the attainment of a social escape function for the hits is that during the period of skill loss characteristic of Rett syndrome, N was involved in a number of Portage teaching sessions and similar activities. It is possible therefore that adult contact was frequently paired with instructional demands that became increasingly aversive as her skills deteriorated. Consequently adult contact itself became aversive.

~_(~6 OllvCr et aL

In this study during social interactions the rate of N's flicks initially increased, and then decreased at the point at which hits dramatically increased. These data, and the scalloped curve indicative of fixed interval escape responding for hits but not flicks (see Figure 2), strongly suggest that previously harder flicks were differentially negatively reinforced by escape from social interactions and eventually became hits. It seems likely that differential negative reinforcement of harder flicks and eventually hits occurred because these behaviors comprised more aversive stimuli to others (adults) than m/td flicks and were thus more likely to elicit escape behaviors from others (i.e., discontinuing the social interactions) which terminated the SIB (Cart & Durand, 1985b; Ferster, 1961; Murphy, 1980; Oliver & Head, 1990). This process of response shaping is similar to that described by Hall and Oliver (1992) for intermittent reinforcement of SIB and differential reinforcement of higher rate SIB and demonstrates how more severe SIB may develop.

The features of Rett syndrome described above, therefore, acted as predisposing factors for the hits to develop and mediated the operant processes of negative reinforcement of hits (i.e., presentation of reinforcement and differential reinforcement) by establishing social interactions as aversive. This interpretation of interactions between features of a syndrome and operant reinforcement challenges the notion of a unitary cause of S1B both within and between individuals. Conversely, it emphasizes the role of different factors that may be construed as determinants that predispose the development of SIB and mediate the operant processes governing the maintenance and development of severe self-injury.

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